In recent decades, multiple biological markers have been discovered which demonstrate a high specificity in determining tumor phenotypes. There has
been much debate on their role in differentiating the origin of metastasis. This article presents a clinical case of peritoneal carcinomatosis arising from
breast carcinoma. The immunohistochemical method was used to determine the tumor phenotype. For this purpose, the markers of gastrointestinal,
ovary and breast tumors were utilized, including CDX2, PAX 8, WT1, CK7, CK20, Mammaglobin, and GCDFP-15. The findings of molecular biological
phenotyping confirmed the diagnosis of primary multiple malignant neoplasms with peritoneal carcinomatosis arising from breast carcinoma. More
accurate identification of tumor phenotypes plays an important role in predicting the disease outcome and specifying indications for prescribing
anticancer drug treatments. The described clinical case demonstrates that molecular biological phenotyping can be used as a proving method for
identifying the origin of peritoneal carcinomatosis. The development of a panel of markers for different tumor localizations will allow a reliable
detection of the source of peritoneal carcinomatosis. In cases of diagnosing peritoneal carcinomatosis from tumors with an undetected primary
lesion, such panel of biological markers will help to reduce the time of examination and improve the quality of medical care.